The apt-based package managers and update managers in Ubuntu and Linux Mint will open multiple connections to repository servers in order to download several packages simultaneously. Unfortunately, the multiple connections cause our old DSL modem (a Westell 6100) to hang. At a former workplace, similar problems occurred with the corporate firewall.
The fix is decribed in this post.
As root, create the file
/etc/apt/apt.conf.d/75download with the following single line:
Then the package managers will attempt to download only one file at a time, reducing the stress on the DSL modem. The downside is that downloads will be slightly slower, but since our DSL connection is already quite slow, this is barely noticeable, and certainly preferable to constantly having to reset the modem during software updates.